1. Field of the Invention
This invention relates to a process for the production of aldehydes by the rhodium-catalyzed hydroformylation of olefins; and particularly to an improvement in such a process wherein unreacted olefin and product aldehydes are recovered from internal and vent gaseous streams by absorption with catalyst solution.
2. Background of the Invention
Processes for forming oxygenated products such as aldehydes by hydroformylation of an olefin with carbon monoxide and hydrogen are well known in the art. The aldehydes produced correspond to compounds obtained by the addition of a carbonyl group to an olefinically unsaturated carbon atom in the starting material with simultaneous saturation of the olefinic bond. Such processes are generally known in the industry by various names such as hydroformylation or oxo processes, reactions, or syntheses and/or oxonation.
Early prior art hydroformylation processes employed cobalt octacarbonyl catalysts. Disadvantages of such processes included the requirement for high operative pressure and an inability to obtain products with an high normal to branched-chain isomer ratio.
A significant improvement in oxo process technology was disclosed by R.L. Pruett and J.A. Smith in U.S. Pat. No. 3,527,809. Pruett & Smith's process is characterized by a high normal to branched-chain aldehyde isomer ratio obtained at a high yield and at low temperatures and pressures by contacting an alphaolefin with certain rhodium complex catalysts in the presence of hydrogen and carbon monoxide under a defined set of variables. The variables included (1) the rhodium complex catalyst, (2) the olefin feed, (3) a triorganophosphorus ligand and its concentration, (4) a relatively low temperature, (5) relatively low total hydrogen and carbon monoxide pressure and (6) the partial pressure exerted by carbon monoxide.
Further improvements have been made over the years with regard to, e.g., the hydroformylation reaction solvent (see U.S. Pat. No. 4,148,830); the use of a gas recycle stream to control the liquid body in the hydroformylation reactor (see U.S. Pat. No. 4,247,486); controlling the hydroformylation reaction conditions to minimize rhodium complex catalyst deactivation (see U.S. Pat. No. 4,277,627); the use of alkyldiarylphosphines to improve the stability of the rhodium complex catalyst (see U.S. Pat. No. 4,260,828); the reactivation of deactivated rhodium complex catalysts by concentration and optional oxygen and/or base treatments (see U.S. Pat. Nos. 4,297,239 and 4,374,278) and the use of reactors in series (see U.S. Pat. No. 4,593,127).
Regardless of whether a liquid or gas recycle process is employed, gaseous purges or vent streams exist. These streams are in some cases necessary to allow inerts and excess hydrogen to escape from the system while in other cases they arise merely as a consequence of certain processing steps and are not really necessary to the overall operation of the system. Because the content of these purges resembles product streams, it is possible to apply conventional recovery technology in an attempt to enhance overall process efficiency. However, often these purges are allowed to escape primarily because their value does not justify the cost of installing and operating compressors, chillers, or other complicated recovery equipment even though they contain significant quantities of unreacted olefin, product aldehydes and alcohols.
The use of techniques, often referred to in the art as "scrubbing" techniques, to recover components from mixed gas streams is well known. Generally, a gas stream is contacted with a suitable liquid solvent in a countercurrent fashion, and portions of the gas are selectively absorbed into the liquid solvent. The resulting liquid solution is normally taken to another piece of equipment where the dissolved gases are separated (i.e., desorbed) from the liquid solvent. Various techniques may be employed to accomplish this separation, with distillation being a common example. The solvent may then be recycled to the gas-liquid contactor.
In general, such a scrubbing operation involves at least two major pieces of equipment the primary gas-liquid contactor and the equipment to separate the dissolved gas from the liquid. In addition, various pieces of minor equipment are also involved: pumps, condensers, heat exchangers, systems for making up for solvent losses, storage tanks for the solvent, etc. Often other major equipment is also required to separate and recover the various components of the desorbed gas stream. It is generally appreciated that significant energy costs are associated with the employment of this equipment.
The operation of such a scrubbing system with a typical organic liquid solvent can be rather complicated, and expensive. In addition, it introduces a foreign material--the solvent--which will tend to contaminate the recovered gas and infiltrate the basic process if the recovered gases are recycled. In the case of rhodium-catalyzed hydroformylation processes, even rather minor contamination of the rhodium-complex catalyst by such solvents can have serious consequences.
The prior art describes various techniques to recover components from oxo-process vent streams. For example, West German Offenlegungsschrift 3102281A1 relates to a method for recovery of unreacted raw materials in the off-gas from a high pressure oxo process. Specifically, the method relates to the hydroformylation of propylene by simultaneously operating a high pressure oxo reactor employing a cobalt-based catalyst and a low pressure oxo reactor employing a rhodium-based catalyst, characterized by the introduction of waste gas resulting from catalyst recovery from the high pressure reaction, which still contains considerable amounts of unconverted propylene, carbon monoxide and hydrogen, into the low pressure reactor. An alternative approach was taken in U.S. Pat. No. 4,533,755 wherein the off-gas from a lower pressure rhodium-catalyst system is compressed and converted in a high pressure cobalt-catalyst system. These disclosures illustrate the complexity and expense of methods which address the recovery of unreacted materials from vent streams. U.S. Pat. No. 3,455,091 discloses a process for separating product aldehydes (particularly n-butyraldehyde and iso-butyraldehyde) from the off-gas formed in an oxo process (i.e., reaction of an appropriate olefin with hydrogen and carbon monoxide in the presence of cobalt carbonyl) by scrubbing the off-gas with a solvent which comprises a high boiling point oxonation product or a high boiling point hydrogenated oxonation product. The off-gas is said to consist essentially of the product aldehydes and carbon monoxide and hydrogen together with small amounts of saturated and unsaturated hydrocarbons having two to four carbon atoms and small amounts of inert gas, such as nitrogen. The aldehyde content of the off-gas varies depending upon the composition of the oxonation product and on the temperature and pressure at which flashing is carried out. The scrubbing solvent used is a high-boiling oxonation product having a boiling point of advantageously more than 95.degree. C., preferably more than 150.degree. C. The preferred solvents, consisting essentially of higher aldehydes formed by aldol condensation of lower aldehydes, acetals, carboxylic esters and higher alcohols, may be obtained as the distillation residue of the processing of the oxo reaction mixture. However, it is also disclosed in U.S. Pat. 4,455,091 at column 2, lines 39-41 that the -main product of the oxo reaction is also suitable provided its boiling point is above 95.degree. C., for example, butanol or n-propanol.
It is further disclosed that the scrubbing solvent containing the aldehydes may be processed together with the bulk of the product obtained in the oxo reaction. During the scrubbing of the off-gas, the solvent is advantageously at room temperature, for example at 15.degree. C. to 25.degree. C., with particularly good results obtained when the solvent is kept at 5.degree. C. to 10.degree. C. Scrubbing of the off-gas is in general carried out at pressures of 0 to 30 atmospheres gauge.
U.S. Pat. No. 2,748,167 discloses a process for preparing oxygenated products such as butyraldehyde by the reaction of an olefin (e.g. propylene), carbon monoxide and hydrogen in the presence of a cobalt catalyst. The patent provides for the escape from the reactor to a condenser of unreacted process gas and vaporized product materials such as normal- and isobutyraldehydes and other products, as well as butanol. The resulting gas and condensate mixture is then passed to a liquid-gas separator to separate the liquid portion from the process gas (primarily unreacted carbon monoxide and hydrogen) which is reintroduced to the reactor. The patent teaches the removal of vent gas from the system in order to reduce the build-up of inerts in the reactor space, thus maintaining the desired synthesis gas composition. The vent gas may also be fed to an alcohol scrubber to recover the olefin portion of the gas.
The liquid removed from the liquid-gas separator, containing crude product and dissolved olefin, is passed to a pressure distillation column to separate and recover olefin for return to the reaction. The crude product may be subjected to subsequent operations, such as distillations, to obtain the various aldehydes in purified form.
U.S. Pat. No. 4,210,426 discloses that when propene (i.e. propylene) is subjected to hydroformylation, gas mixtures are obtained in addition to liquid reaction products such as n-butyraldehyde, iso-butyraldehyde, n-butanol and iso-butanol. These gas mixtures are said to consist of the unconverted components (carbon monoxide, hydrogen and propene) and of propane. It is disclosed that they had been previously burned as off-gases or converted, together with by-product iso-butyraldehyde, to the reactants necessary for the hydroformylation, but that such conversion is no longer economical. It is also disclosed that propene and propane may be recovered from these gas mixtures by condensation, by extractive distillation or by absorption using a recovery agent. However, the patent discloses that these processes are unsuitable and uneconomic because they are eithher expensive or have the considerable disadvantage that the recovered gases require careful purification before being reemployed in the hydroformylation.
The invention described in the U.S. Pat. No. 4,210,426 is the use of the liquid hydroformylation products to absorb propene and propane from the off-gases. It is said that these absorbents offer the great advantage that, after desorption, small amounts of the absorbents do not have to be separated off but can be recycled to the hydroformylation reaction together with recovered propane. The preferred absorbents include iso- or n-butyraldehyde, although iso-butyraldehyde is particularly used since it is more stable to heat and also since n-butyraldehyde is the valuable main product of the hydroformylation.
High pressures (i.e., 10 to 60 bars) and low temperatures (i.e., 0-50.degree. C., preferably 20-40.degree. C.) are said to increase the absorption effect it possible to obtain a gas product, largely free of propene and propane, essentially consisting of carbon monoxide and hydrogen which can be recycled to the hydroformylation reaction. The absorbed propene and propane is desorbed (i.e., separated from the absorbent) in a known manner; for example, propene only may be separated first by combining the desorption with a fractional distillation, leaving propane in the absorbent and preferably recycling the separated propene to the hydroformylation reaction. Subsequently, the remaining propane in the absorbent may be separated by distillation and the absorbent recycled to the hydroformylation.
Although U.S. Pat. No. 4,210,426 employs hydroformylation products as a scrubbing solvent and thereby avoids contamination of the catalyst solution, it teaches a standard scrubbing arrangement to recover the propylene and propane from the reactor vents by desorption from the scrubbing solvent. As with typical scrubbing arrangements, and in direct contrast to the present invention, a desorption step is used in the process disclosed by U.S. Pat. No. 4,210,426.